on_import = True causes the plugin to run just before the file is added to the database, but not before metadata is read from the file. If you want to change metadata reading, create a metadatareader plugin.
Getting the path to the calibre install directory is platform and installation method dependent. Here is some code to find the path to the plugins subdirectory ina calibre install
plugin_path = os.path.join(os.path.dirname(sys.executable), 'plugins')
plugin_path = os.path.join(getattr(sys, 'frameworks_dir'), 'plugins')
plugin_path = os.path.join(getattr(sys, 'frozen_path'), 'plugins')
Note that you shouldn't need to know the path to the calibre install directory in your plugins.